Silica brick is a very important material which is used as refractory for building up and repairing industrial furnaces, mainly coke ovens and hot blast stoves. It is well known that silica, which is the main component of siliceous stone material as the principal material of silica brick, is changed into cristobalite and tridymite due to crystalline transformation in the course of firing for production of the silica brick. Silica brick having a large content of non-transformed silica is unfavorable because it exhibits extraordinary expansion in use as the refractory of industrial furnaces, resulting in impairment of stability of the furnaces. Therefore, the extent of transformation of silica is one of very important factors which have to be considered in designing an industrial furnace in regard to selection of material and evaluation of adequateness of the use of the selected material.
A general process for producing silica brick comprises the steps of pulverizing a siliceous stone material, such as composite quartzite (red-white or blue-white), into particles of appropriate size and mixing the particles with lime (CaO) which is an additive also serving as a binder.
A silica brick for use as a material of glass kiln is also known, which contains, for the purpose of preventing generation of frost-like matter on the brick surface of the upper structure of a glass melting cell, 0 to 1.5% by weight of CaO and 0.3 to 1.5% by weight of K.sub.2 O, based on the weight of silica brick, the sum of CaO and K.sub.2 O being not more than 1.5% by weight (see JP-B-49-16087 (the term "JP-B" as used herein means an "examined Japanese patent publication")). It has also been proposed to add Fe, Fe.sub.2 O.sub.3, Cu.sub.2 O, TiO.sub.2, Si, etc.
The degree of ease of transformation of silica in a siliceous stone material into cristobalite and tridymite on heating tends to be affected by such factors as crystal grain size of silica and impurities in the siliceous stone material, and also varies depending on the place where the siliceous stone material is produced.
In general, a greater crystal grain size or a smaller impurity content tends to hamper the transformation of silica.
The present inventors have previously proposed an excellent additive which promotes transformation of silica in a siliceous stone material of the type in which transformation of silica is not easy, and a method of producing silica brick by using the additive as disclosed in U.S. Pat. No. 5,310,708 (corresponding to JP-A-5-4864 (the term "JP-A" as used herein means an "unexamined published Japanese patent application")). The method disclosed is characterized by adding from 0.2 to 5% by weight based on a siliceous stone material of Na.sub.2 O--CaO--SiO.sub.2 fused and solidified material to the siliceous stone material. This additive reacts with silica in the siliceous stone material to promote transformation of the silica into cristobalite and tridymite, thereby producing silica brick having a markedly reduced content of residual silica, even when starting with a siliceous stone material in which transformation of silica is not easy. As a result of further investigations, the present inventors found that the addition of the Na.sub.2 O--CaO-SiO.sub.2 fused and solidified material in the above specified amount results in an increase in amorphous glass content of the resulting silica brick. A large amorphous glass content in silica brick causes reduction in resistance to creep during long-term use as structural material of a coke oven or a hot blast stove.
On the other hand, too large a cristobalite content in silica brick makes the structure instable. Within a working temperature range of silica brick in coke ovens or hot blast stoves, the range of from 800.degree. to 1400.degree. C. is a region in which tridymite is stable. Therefore, cristobalite in silica brick used in the furnace is transformed to tridymite during use so that the brick after use is silica brick mainly comprising tridymite. The transformation from cristobalite to tridymite during use of a coke oven or a hot blast stove is accompanied by expansion, which makes the structure instable. The present inventors have analyzed silica brick after use and, as a result, found that silica brick mainly comprising tridymite is a preferred material of industrial furnaces.
The present invention has been reached based on the above findings.